Legal Question in Immigration Law in California

In short, this is a huge problem. If her husband was still legally married to his first wife at the time he married her, then her greencard is technically invalid, since bigamy is prohibited in the United States.

Her husband, however, has no say as to whether or not she files for naturalization, so I don't know why that stopped her from filing before.

If she files for naturalization now, as long as USCIS does not know or find out about the issues with her husband, then her case could theoretically be approved. (Although I cannot condone fraud or advise her to file at this point).

She is better off speaking with a Family Law attorney in her State of residence first, with regards to the marriage, divorce, spousal support, property division, & other legal issues regarding her marriage.

If she looses her greencard, she will have to return to her home country. You cannot petition her, since she is only related to you by marriage. Furthermore, a sibling petition takes many years to become current, and Congress is discussing terminating sibling petitions anyway.

Good luck.